Vehicular on-board power supply systems, for example automotive power supply systems, usually use self-excited three-phase alternators. The three-phase alternator has a three-phase rectifier network, preferably a full-wave rectifier network connected thereto to supply d-c power to a battery to keep the battery charged and supply loads therefrom. Usually, the rectifier includes a separate branch of a rectifier array which is provided to supply d-c to energize the field of the alternator. The field current itself is controlled by a voltage regulator. In an electronically controlled system, a power switching transistor is connected in series with the field, the power switching transistor forming part of the voltage regulator and being gated ON or OFF to supply current pulses to the field. The field itself is bridged by a free-wheeling or bypass diode to maintain current flow through the field during the OFF periods of the switching transistor. The positive terminal of the excitation voltage is usually referred to as the D+ terminal, the negative terminal as the D- terminal, and the connection point between field winding and switching transistor as the DF terminal.
Upon operation of the ignition switch, the supply to the battery is connected thereto and, at the same time, a charge indicator is activated. The charge indicator, usually, is a charge control lamp connected between the positive terminal of the vehicle battery, usually referred to as the B+ terminal and the D+ terminal. This charge control lamp indicates if current is flowing from the battery to the rectified output from the alternator, that is, indicates discharge of the battery and, additionally, is an indicator of trouble in the electrical system of the vehicle itself. The charge control indicator, typically an incandescent lamp, is customarily placed on the dashboard of the vehicle at a suitable location, frequently arranged together with other indicator lamps or apparatus.
The charge control lamp indicates if the battery is not being charged. Discharge of the battery, particularly for an extended period of time, must be indicated to the operator since, if no electrical power is available, ignition to the engine will fail. The battery can be damaged by rapid discharge. Batteries are sensitive, however, not only to excessive discharge, or to complete discharge state, but also to excessive charge. Supervisory apparatus which indicate overcharging of the battery have also been proposed. Such supervisory apparatus usually operates in the form of a bi-metal switch and prevents overcharging of the battery by interrupting connection to the alternator, or short-circuiting of the alternator. Such switches are so arranged that, after their response, it is no longer possible to operate the vehicle under emergency conditions.
Supervisory apparatus has also been proposed to indicate both overcharging and undercharging of a battery, usually in the form of a center-positioned ammeter. Use of pointer-type instruments on board of a vehicle is undesirable; the instruments cannot be made to indicate accurately due to the mechanical shocks and vibrations which are inherent in automotive applications. Use of pointer-type instruments additionally is undesirable since operators frequently overlook checking such meters, so that malfunction in the current supply system of the vehicle is not called to the operator's attention with sufficient emphasis.
Electronic supervisory control apparatus has also been proposed which, in case of trouble, cause a control lamp to light and to provide an indication to the operator that there is trouble in a current supply system. Such electronic apparatus usually is an additional or accessory item which requires additional wiring and connection. Output terminals of the electronic apparatus and connected to such an indicator usually must be well insulated to prevent any possible malconnection or short circuits, which increases cost and assembly of the vehicle and the electrical system therefor.
Determining the charge state of batteries, for example the type of secondary batteries used in automotive applications, is known; insufficient charge of the battery can occur due to overloading of the on-board vehicular network, or due to malfunction of the alternator, or of the voltage regulator connected therewith; overcharge of the battery can occur due to malfunction in the voltage regulator.